Thanks chaps. Yes good point, de-bouncing the switches. I know the problem but it hadn't occurred to me in this context.
Could you explain a little more about using a 555 to trigger pulses to make the clock pin HIGH/LOW? If you were to control the 555 with a switch would that switch itself not need de-bouncing also?!
If I understand things correctly, is it only the clock pin that needs de-bouncing because even if the data pins bounce they will settle on the intended value eventually and the next bit isn't considered until the clock pin advances the 595 to it?